1. Field of the Invention
This invention relates to an apparatus for measuring particles in liquid, and more particularly to an apparatus for measuring particles in liquid in which a laser beam is projected into a flowing liquid and light scattered from particles present in the liquid is evaluated to measure particle size, number of particles and other characteristics of the particles.
2. Description of the Prior Art
The prior art includes techniques in which light is projected into a measurement zone and the amount of transmitted light, scattering characteristics and the like are measured to determine the size, number and other characteristics of particles in the zone.
For example, while this technique is employed in monitoring particulate impurities in pure water, since such particles in pure water tend to be small and present in low concentrations, the particles are difficult to monitor. For this reason, in order to increase the intensity of light scattered from particles, an incident beam from a laser light source or the like is conventionally collected in a small zone, creating a measurement zone of high light intensity in the measurement cell, and then the scattered light from particles passing through this zone is received (see, for example U.S. Pat. No. 4,830,494).
In a particle measurement apparatus which projects laser light at particles and analyzes light scattered from the particles, the direction from which light scattered from the particles is received is important as is the shape of the measurement section through which the particles pass in the measurement cell. Because the distribution of laser light intensity exhibits a Gaussian distribution, conventional apparatus for measuring particles in liquid employs a "90-degree lateral light receiving system" in which light scattered from particles illuminated with laser light is received in a direction nearly perpendicular to the axis of projection of the laser beam in order to improve the efficiency of particle measurement.
This system employs an arrangement in which the direction of particle travel is colinear with the optical axis of a light receiving lens or in which the direction of particle travel lies in the plane defined by the optical axis of the laser and the optical axis of the light receiving lens and crosses the optical axis of the light receiving lens at an angle in the range of 20-70 degrees. Slits are disposed on the imaging surface of the light receiving lens so that scattered light from the particles which pass through areas of high light intensity is selectively received to obtain good particle size resolution.
Furthermore, in order to force particles to travel in a specific direction through measurement cells employing this sort of system, there are methods in which sheath flow is employed to establish cylindrical flow through the measurement zone, and methods in which a stirrer is used with a cylindrical wall to establish a rotating flow.
In the above method using sheath flow, the high rate of flow required to establish the cylindrical flow becomes a problem. The method using rotating flow, on the other hand, disadvantageously has the lack of a way of verifying that liquid in the cylindrical section is adequately circulated through the inlet and outlet pipes in the cell with the cylindrical section used to establish the rotating flow, and has the possibility that the same particle would repeatedly pass through the measurement zone.
As described above, when forcing particles to travel in a certain direction through the measurement zone, if the flow of particles assumes a turbulent flow state rather than a laminar flow state, the direction of travel of particles cannot be set to a direction along the optical axis of the light receiving lens or to a direction parallel to the plane defined by the optical axis of the laser and the optical axis of the light receiving lens, so that good particle resolution cannot be obtained.